Send Stacky a photo first and say what to do with it after #203

Story

As a family member, I want to send Stacky a photo and then say what to do with it in my next message, so that "Stacky, put this on the shopping list" works with apps that send a photo without a caption.

Context

Since #202 a photo reaches Stacky when its caption starts with Stacky's name. In a room with several people a photo without a caption carries no name, so nanobot's gate drops it (stacklets/agent/runtime/sitecustomize.py, name and thread triggers). The text that follows addresses Stacky, but the photo it refers to was never part of the conversation. Element X sends captions; older clients and quick sharing from the gallery often do not.

Proposal

When a message addressed to Stacky arrives, the same person's most recent photo in that room from the last few minutes, if it was not addressed to anyone, goes to the model with it. A photo nobody refers to stays unseen.
Out of scope: documents and PDFs (the archivist's job), photos from another person.

Definition of done

• A photo without a caption, then "Stacky, put this on the shopping list" from the same person, adds the items on the photo
• A photo from Marge followed by "Stacky, ..." from Homer does not attach Marge's photo
• A photo older than the window is not attached
• Tests state which photo goes with which message